Device for testing magnetic materials



Sept 7, 1948. r. r. GoLDsMlTl-l, JR., Erna. 2,448,794

I DEVICE FOR TESTING MAGNETC MATERIALS Filed June 4, 1943 IAL( few'

33 y JLmMfMJMm/VENTORJ wv @37M ITTORNEY Patented Sept. 7, 1948 y auam t y nevica Fon rEs'rmo mamme Ma'raams Thomas T. Goldsmith, Jr., Cedar Grove, and Eatle Ray Mann, Upper Montclair, N. J., aav signora to Allen B. Du Mont Laboratories, Inc.,

Passaic, N. J., a corporation of Delaware Application Juno 4, 1943, Serial No. 489,852 (Claims. (Cl. 175-183) This invention relates to a device for testing materials. It is particularly useful in testing materials byexamination of their magnetic properties or permeabiiities. It is also useful for testing dielectric propertiesof materials.

In carrying out the invention, a high frequency koscillator is used to produce. magnetic flux through a predeterminedregion. Modifying or modulating magnetic ilux may be supplied to the 4 The invention may be understood from -the de-v scription in connection with the accompanying drawings, in which: l

Fig. 1 is a block device; 4

Fig. 2 is a diagram of onel embodiment of electrical connections; and v Figs. 3, 4 and 5 are illustrative cyclographs obtained with the device.

In'Flg. 1 of -the drawings,V the several blocks are labeled to designate what they'represent.l

In Fig. 2,-reference character I indicates a source of alternating current which may be 60 v cycles per second. It is connected to the primary 2 of a transformer that has two secondaries l andl. .The coil 4 is groundedgnear its center. One end thereof is connected to resistor I, that is providedwith a sliding contact i connected by lead 1 toone. terminal of the horizontal deflecting system H of the oscillograph O, the other terminal thereof being grounded. 'I'he other end of coil 4 is connected through condenser 8 to the lead I "Thesliding` contact 6 on resistance i,

and condenser 8 provide a phase-shifting device forthe voltage applied to the system H of the oscillograph O.

'I'he secondary 3 is connected to a coil I0 that is adapted. to be coupled Vwith the coil I I through the medium of the specimen to be tested. One

end of coil Il is connected to a resistance I2 that is provided with an adjustable contact I3 which is coupled by condenser I to the other end of coil-I I. The condenser I5 is essentially in parallel diagram of elements of thev 2 withY the coil II to produce resonance. A lead Ii extends from oneend of coil I I to the grid of tube Il, Ywhich is providedy with a grid leak Il. The cathode of this tube is biased by resistance Il.

The plate of tube I1 is coupled by condenser 20 to the grid of tube 2l, which is provided with grid leak 22. The cathode of tube 2l is also biased by resistance, as shown at 2l.

The grid of tube Il is connected through adinstable resistance 25 and condenser 26 to the. plate of tube 2l. Resistances 2l and 2l are provided in the usual way between a source of positive potential and the plates of tubes Il and II. 'I'he tubes Il and 2| and the connections described constitute an oscillator "with regenerative feedback.

The plate of tube I1 is coupled through condenser 30 and lead Il to one terminal ofthe vertical deiection system V of the oscillograph 0, the other terminal thereof being grounded.

The operation is as follows:

Low frequency alternating current is applied from source I through transformer ,2-3 vto the coil It, and potential is applied to the plates of tubes I1 andZI.. Low frequency potential is also applied to coil l. Under these conditions low frequency waves are applied through lead 'I to the horizontal deilecting system H of the oscillograph and high frequency waves from the oscillator are applied through lead II to the vertical deflecting system V thereof. The coil Il reacting with coil I I causes the high frequencyoscillations or waves indicated at 32 in Fig. 3 to be modulated by the low frequency wave as'indicated by the curves Il when no magnetic material is in the coils Il or II. l

When a standard specimen of material that has magnetic properties is placed as 'a core in the coilsv Il and II, the increase of ilux causes the cyclogram to change, for example. into that shown in Fig. 4. This cyclogram shows at a and b that there are -two minima per low frequency cyclein the amplitude of the oscillator whenthere is magnetic material i n the coils Il and II. 'I'he second minimum is more pronounced the more saturation there isin the permeable material.

When a candidate specimen of like shape and size as the standard is to be tested it replaces the Vstandard specimen as a core in the coils III and Il. The cyclogram changes if the speci--d men being examined differs in magnetic propertiesfrom the standard. For example, Fig. l5 show's a cyclogram obtained using a' specimen of steel like that used in obtaining the cyclogram of Fig. 4, except that the carbon content and therefore the permeability was different.

While a grid-modulated self-oscillating circuit has been shown, other conventional methods of modulation such as, for example, plate modulation of an amplifier tube carrying oscillating signals from any convenient frequency source, may be used in this invention.

In the specific embodiment of Fig. 2, when the sample under test is inserted in coils I and III the low frequency signal from coil I0 is coupled to the coil I'I. At the same time the sample under test reacts directly to control the amplitude of the high frequency oscillations. The sample under test can be made to exhibit its low frequency saturation effects as a modulation of a higher frequency amplitude.

In practice, the coils I0 and II and the condenser I are chosen to match the specic characteristics of given samples. The equipment is arranged for rapid substitution of other sets of coils and condensers to match new samples.

In order to obtain the most sensitivity of the pattern, such as indicated by Fig. 4 or Fig. 5, it

is desirable to adjust the strength of low fre-A quency current applied to coil I0 and to adjust the amount of feed-back contributed by resistor 25. 'Ihe adjustment of resistor 5 shifts the low frequency phase of the horizontal scanning so as to produce easily interpreted patterns of the minima in the cyclograms.

The choice of the resonant frequency determined by coil II and condenser I5 iniiuences the nature of the c'yelogram pattern. If the entire body of a sample is to be inspected down to the center than relatively low frequencies vare employed. `For example, generator I may be 60 cycles and the resonant frequency determined by coil II and condenser I5 may be 1000 cycles.

However, if the surface conditions are of mainv interest, then one employs a coil II and condenser I5 which resonates at a high frequency, such as 20,000 cycles, thus detecting skin effects of the specimen.

While the application of the modulating signals by means of the coupling from coil IIJ to coil I I is illustrated in Fig. 2, it is desirable in some cases to so apply the modulatingsignal that the low frequency flux is not applied directly to the sample under test. For example, a modulating voltage may be inserted in the plate supply of tubes I1 and 2l by connection of a transformer secondary between the point marked and the common lead for resistances 28 and 29. The modulating signals may be inserted elsewhere in the circuit.

The cathode-ray tube provides a convenient means of observing and interpreting the modula tion effects. However, any other means of measuring and interpreting the modulation may be employed, suchv as, for example, a rectifier and meter.

Where it is desired to measure the dielectric properties of a material, the material is used as the dielectric medium of the condenser I5 so that| Pieces of material may also be tested in accordance with this invention which are slightly different in chemical content. Also pieces having slightly different sizes, but otherwise uniform as to chemical composition and heat treatment, may be tested.

What is claimed is:

1. A device for testing specimens of magnetic materials by examining their magnetic proper.- ties, which comprises a high frequency vacuum tube oscillator having an inductance coil in the oscillator circuit thereof with a condenser in parallel with said coil, one end of said coil and the corresponding side of said condenser lbeing grounded, an inductance coil aligned with said first named coil, said coils being adapted to be coupled magnetically by the specimens of material to be tested, means to lproduce low frequency oscillations in said second named inductance coil and an oscillograph having oneset of deflection plates connected to the output of said high frequency oscillator and another set of deflection plates connected to-said means which produces low frequency oscillations whereby a visible pattern is produced on said oscillograph which is characteristic of the magnetic properties of said material.

2. In a device for testing specimens of magnetic material by alternating magnetic flux, a high frequency oscillator which comprises two vacuum tubes each having its plate coupled to the control grid of the other, a tank circuit comprising a condenser and an inductance `coil in parallel connected to the control grid of one of said tubes, said coil being adapted to 'receive a specimen of the metal to be tested, a source of low frequency current having an inductance coil connected thereto, two inductance coils located in inductive relation to said last named coil, an oscillograph having a deflection system to which one of said two coils is connected and having a different deflection system to which the 'output of said oscillator is connected, and a coil having its ends connected respectively to the ends of said other one of said two coils and being aligned with said tank inductance coil and also being adapted to receive specimens of the material to be tested.

3. The device of claim 2 in which a phase shifter is located between said first named deflection system and the coil to which it is connected.

4. The device of claim 2. in which a variable resistance is located in said tank circuit.

THOMAS T. GOLDSMITH, Jn. ESTLE RAY MANN.

REFERENCES CITED The following references are of record in the file of this patent: 

